How to access Pyodide APIs to customize init

I've read through the docs, a bunch of issues, and played around with a few of the templates (the GitHub starter and "jupyter lite" commands), but can't figure out how to do this, so maybe an area for docs improvement.

How do you access the Pyodide APIs to configure the pyodide kernel before user environments (REPL, Notebooks) are made available in JupyterLite?

In this particular case, a package I want to use is only shipped as native binaries and doesn't have a pyodide compatible build available, but can be easily provided by a JavaScript shim and registered via `pyodide.registerJsModule` (see https://pyodide.org/en/stable/usage/faq.html#how-do-i-create-custom-python-packages-from-javascript ), so I want to run this before-hand in the kernel at startup. How should this be done?